1. Field of the Invention
This invention relates to a method and apparatus for removing arsenic from drinking water and in particular to removing arsenite (As+3) and arsenate (As+5) from a water system at a point of entry or point of use by first converting arsenite (As+3) to arsenate (As+5) and then removing the arsenate.
2. Description of Related Art
Arsenic is a naturally occurring substance found in a variety of combined forms in the earth. Arsenic is a Group 5A nonmetal with the more common valences of xe2x88x923, 0, +3 and +5. Arsenite (As+3) and arsenate (As+5) are the most common forms found in drinking water and waste water streams. The United States Environmental Protection Agency maximum contaminant level (MCL) for arsenic is 50 parts per billion (ppb). The World Health Organization""s (WHO) standard MCL for arsenic is 10 ppb.
The United States Environmental Protection Agency (EPA) has stated that arsenic in drinking water causes cancer in humans and that the amount of arsenic in water at the current allowed concentration may equal that caused by smoking cigarettes. There are negotiations to reduce the current maximum concentration limit of 50 ppb to as low as 10 ppb. Therefore, water providers have a need for an economical safe method to remove arsenic from drinking water. Further, residential homes obtaining water from wells have a need for a low cost, safe, and efficient point of entry or point of use arsenic removal system.
Technologies which have been used for removal of arsenic include the following: (1) adsorption onto activated alumina within a fixed bed contactor; (2) complexing arsenic with hydrous metallic floc, previously aluminum and iron hydroxides or oxyhydroxides, in conventional water treatment plants; (3) sieving the metal from water by membrane technologies such as reverse osmosis; and (4) electro-dynamic processes such as electrodialysis.
However, most technologies for arsenic removal are plagued by the basic difficulty of removing arsenite (As+3). The more successful techniques are ones that have been used on large municipal supplies, but they are not practical for residential application because of space requirements, the use of dangerous chemicals, frequent monitoring and expense. The two most common techniques for residential water correction have been reverse osmosis (RO) and activated alumina. Activated alumina requires the use of caustic chemicals and a very large volume for the high flow rates available with this invention. RO is no longer certified as an arsenic removal technique because of its inability to reduce arsenite (As+3) significantly.
U.S. Pat. No. 5,368,703 issued Nov. 29, 1994 to Michael D. Brewster describes a process and apparatus for removing arsenic from wastewater. The process comprises the steps of providing a bipolar electrochemical cell 10 comprising a pair of outer electrode elements, passing the wastewater through the cell 10 while applying an electric potential across the electrodes so that the anode and cathode are oppositely charged, positive and negative, producing anodically ferrous ion in the form of an insoluble iron compound, specie, or complex in the ionizing media, and subjecting the media containing the iron compound specie or complex to a mild oxidation sufficient to convert the ferrous ion present therein to ferric ion and to oxidize the arsenic therein to +5 valence specie to form ferric arsenate and an hydroxy ferric oxide-arsenate complex which are insoluble and removable from the media in a clarifier. However, this method is incumbered by the need for an electrochemical cell, multiple chemical feed pumps, the addition of chemicals requiring precision monitoring and necessitating storage and repressurization of the water.
U.S. Pat. No. 5,575,919 issued Nov. 19, 1996 to Peter F. Santina describes a method and system for removing toxic substances from drinking water such as arsenic by the use of finely divided metallic iron in the presence of powdered elemental sulfur or other sulfur components such as manganese sulfide followed by an oxidation step to effect arsenic recovery as a precipitate which is separated from the water. However, this method requires a mixing vessel, the creation and use of sulfur modified iron, and the addition of acid as well as other chemicals requiring precision monitoring.
U.S. Pat. No. 5,591,346 issued Jan. 7, 1997 describes a water or wastewater purification process for reducing selenium and arsenic concentrations in contaminated water by the use of a cation exchange resin. Contacting the contaminated water stream with an iron (III)xe2x80x94complexed cation exchange resin in an ion exchange column forms an acid exchangeable iron III arsenate complex immobilized on the cation exchange resin and in effluent stream having reduce arsenic concentration. However, this method is incumbered by the need for a chemical feed pump to inject an oxidizing agent, the addition of an acid to regenerate the resin and the addition of deionized water for rinsing of the resin.
In the above three patents each system requires highly skilled personnel for operation and maintenance on an ongoing basis. Therefore they are not suitable for residential use at point of entry.
Accordingly it is therefore an object of this invention to provide a method and apparatus for removing arsenic from water including arsenite (As+3) and arsenate (As+5).
It is another object of the invention to remove arsenic from water by pre-oxidizing the arsenic to convert arsenite (As+3) to arsenate (As+5).
It is a further object of this invention to provide an arsenic removal system that is suitable in cost, space required, and efficiency for residential environment or commercial application.
It is yet another object of this invention to reduce the concentration of arsenic in drinking water to below 10 ppb and in some applications to a nondetectable level.
These and other objects are further accomplished by a system for removing arsenic from water comprising means connected to the water for converting arsenite in the water to arsenate, and means connected to the arsenite converting means for removing arsenate in the water. The arsenite converting means comprises manganese greensand. The system comprises a solution of potassium permanganate for regenerating the manganese greensand. The arsenite converting means comprises means for controlling the frequency of regenerating the manganese greensand. The means for removing arsenate from the water comprises an anion exchange resin bed. The system comprises a salt solution for regenerating the anion exchange resin bed. The arsenate removing means comprises means for controlling the frequency of regenerating the anion exchange resin bed. The system comprises a plurality of valve means for allowing the water to flow to a user bypassing the arsenic removing system. In an alternate embodiment the means for removing the arsenate comprises a reverse osmosis system.
The objects are further accomplished by a system for removing arsenic from water comprising a first control unit connected to the water, a container of manganese greensand connected to the first control unit for converting arsenite to arsenate as the water passes through the manganese greensand, a container connected to the control unit for supplying potassium permanganate to the manganese greensand for regenerating the manganese greensand, a second control unit connected to the first control unit for passing the water from the manganese greensand container through an anion exchange resin bed for removing arsenate from the water, and a salt tank connected to the second control unit for regenerating the anion exchange resin by passing a salt solution through the anion exchange resin. Each of the first control unit and the second control unit determines the time of regeneration of the manganese greensand and the anion exchange resin respectively. The system comprises a plurality of valve means for allowing the water to flow to a user bypassing the arsenic removing system.
The objects are further accomplished by a method of removing arsenic from water comprising the steps of providing means for converting arsenite in the water to arsenate, and providing means connected to the arsenite converting means for removing arsenate from the water. The method comprises the steps of providing first control means for regenerating the arsenite converting means and second control means for regenerating the arsenate removing means. The step of providing first control means and second control means comprises the step of providing timer means in each of the first control means and the second control means for determining when regeneration occurs. The step of providing means for converting arsenite to arsenate comprises the step of flowing the water through manganese greensand. The step of removing arsenate comprises the step of providing an anion exchange resin bed for the water from said arsenite converting means to flow through. The step of regenerating the arsenite converting means comprises the step of passing potassium permanganate through the manganese greensand. The step of regenerating the arsenate removing means comprises the step of passing a salt solution through the anion exchange resin bed. In an alternate embodiment, the step of removing arsenate comprises the step of providing a reverse osmosis means for removing arsenate from the water from said arsenite converting means.
Additional objects, features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.